1. Field of the Invention
The present invention relates to an epitaxial nano-structure and manufacturing method for growing epitaxy and, more particularly, to a three-dimensional GaN epitaxial structure and manufacturing method thereof.
2. Description of the Related Art
Conventional manufacturing method for growing Gallium nitride (GaN) structure is metal-organic chemical vapor deposition, MOCVD. For forming a thin film on a substrate through MOCVD method, a carrier gas such as hydrogen (H2) or nitrogen (N2) carries a saturated vapor, which is generated by a metal-organic source, to a chamber to mix with other gases and a chemical reaction is then taken place on a surface of the substrate with high temperature in the chamber, so that the thin film with large area is formed on the surface of the substrate by deposition. Namely, the thin film grows on the substrate gradually when the MOCVD method is processed.
In MOCVD, the carrier gases can be selected from hydrogen (H2) or nitrogen (N2). Materials of the substrate can be selected from Gallium arsenide (GaAs), Gallium phosphide (GaP), Indium phosphide (InP), Silicon (Si), Silicon carbide (SiC), or Aluminium oxide (Al2O3)(Sapphire) etc. And desired materials of the grown thin film are usually compound semiconductors of group III-V or II-VI such as Gallium arsenide (GaAs), Aluminium gallium arsenide (AlGaAs), Aluminium gallium indium phosphide (AlGaInP), or Indium gallium nitride (InGaN) etc. Generally, the thin film made of the above semiconductors by deposition is applied to photoelectric elements such as light emitting diode (LED) etc.
However, through MOCVD method, it is difficult to control a thickness and manufacturing quality of the thin film. Moreover, instead of a small-sized thin film of GaN, the thin film of GaN formed by the above MOCVD method is in a large size. Thus, when the large-sized thin film of GaN is utilized to form photoelectric elements, light emitted by the photoelectric elements is liable to have a broad range in wavelength, which causes low luminous intensity.
Another conventional manufacturing method for GaN structure is molecular beam epitaxy, MBE. Apparatus of MBE includes a chamber with ultra-high vacuum, a substrate received in the chamber, a plurality of crucibles, and a gas source, with both of the crucibles and gas source communicating with the chamber. At least one kind of materials for deposition such as a metal element, Gallium (Ga) is put inside the crucibles and then is heated and vaporized to a vapor phase to be sucked into the chamber. On the other hand, gas exhausted from the gas source such as nitrogen (N2) will be excited to a plasma state by illumination of an energy device and flow into the chamber. Hence, the Ga vapor and the plasma N2 can mix together and react to dispose GaN on the substrate.
Taiwan Patent Publication No. 200420492 entitled “III-N NANOSTRUCTURE AND FABRICATION METHOD THEREOF” illustrates an example for using a conventional MBE method to grow GaN structure. A substrate selected from Aluminium oxide (Al2O3), silicon (Si), Gallium arsenide (GaAs), Zinc oxide (ZnO) or glass is used to grow GaN with temperature 500 to 1000° C. A hexagonal nanopillar structure, which has a flat top surface can be applied to nano light emitting diode (nano-LED), nano laser diode (nano-LD), or nano field effect transistor (nano-FET).
However, due to difference of lattice sizes between GaN and material of the substrate such as Al2O3 or Si, dislocation or defect will be happened easily in initial grown structure of GaN on the substrate. For example, there is a difference around 14% between GaN and Al2O3, and around 20% between Si and GaN. Moreover, if the nanopillar of GaN formed by MBE is applied to luminous elements, a connecting portion for connecting with an electrode has to be made on the top of the GaN nanopillar. However, the diameter of the top of the GaN nanopillar is smaller than 1 μm, and thus difficulty in arranging the connecting portion on such small area is high.
Taiwan Patent No. 1300632 entitled “GROUP-III NITRIDE VERTICAL-RODS SUBSTRATE” discloses another example for using the conventional MBE method to grow GaN structure. A buffer layer of SiN or other group III nitrides is initially formed on substrate of materials such as Si, SiC, or Al2O3, and then vertical rods of GaN are formed on the buffer layer.
However, if GaN rods as described above are applied to luminous elements, the diameter of the top of each GaN rod is too small to form a connecting portion thereon. Moreover, the additional and needed buffer layer for growing the GaN structure leads to complicate manufacturing processes and time consuming. Hence, there is a need for an improvement over the conventional GaN structure and manufacturing method.